1. Field of the Invention
The present invention relates to a method of fabricating a superconductive electrical conductor of Nb.sub.3 Sn type.
2. Related Art Statement
Hitherto, it was difficult to put to practical use a superconductive material of an intermetallic compound type, which is said to be superior in various superconductive characteristics to a superconductive material of an alloy type, because of a low workability of the intermetallic compound type superconductive material. However, the intermetallic compound type superconductive material has been led to be widely put to practical use by the development of a fabricating method utilizing a metallic diffusion reaction in which a working is applied to the materials under such a composite condition that the intermetallic compound is not yet formed, and after the working a diffusion heat-treatment is applied to the materials to produce the intermetallic compound.
There is a superconductive material of Nb.sub.3 Sn type as one among the above-described intermetallic compound type superconductive materials, which is high in critical temperature (Tc) and, further, is easy in generation of strong magnetic field. Hitherto, a bronze method and an Sn plating method have been known as a typical method of fabricating a superconductive electrical conductor, in which the Nb.sub.3 Sn type superconductive material is utilized. The former bronze method is arranged such that an Nb filament is disposed in a matrix of a Cu-Sn alloy (bronze) having an Sn concentration desirably on the order of 10 to 15 wt. % to form a composite wire or cable having a predetermined diameter and, subsequently, a diffusion heat-treatment is applied to the composite wire to diffuse the Sn contained in the Cu-Sn alloy, to provide a superconductive electrical conductor having therein the Nb.sub.3 Sn filament. The latter Sn plating method is arrange such that an Nb filament is disposed in a matrix of pure Cu or of a Cu-Sn alloy so as to provide a wire a predetermined diameter, an Sn plating is subsequently applied to an outer periphery of the Cu matrix and, subsequently, a diffusion heat-treatment takes place to diffuse Sn from the plating layer through the Cu matrix, to provide a superconductive electrical conductor having therein the Nb.sub.3 Sn filament.
It has been said, however, that although Nb.sub.3 Sn produced by the diffusion reaction of Nb-Sn represented by the above-described bronze method and the Sn plating method indicates high critical current density (Jc) in an external magnetic field up to 10T (tesla), the Jc value is rapidly decreased in the magnetic field higher than 10 T.
Recently, is has been found that if a third element such as, for example, Ti, Si, and Hf other than Nb and Sn is added into the Nb filament or the bronze matrix, the decreasing rate of the Jc value can be improved in the higher magnetic field range above the vicinity of 10 T. Accordingly, the third element has been added to fabricate the Nb.sub.3 Sn type superconductive electrical conductor. In practice, in the fabrication of the Nb.sub.3 Sn type superconductive electrical conductor containing the third element, the Nb filament which has added thereto the third element and is alloyed is utilized, or the bronze matrix which has added thereto the third element and is alloyed is utilized, or an alloy in which the third element is contained in Sn by a diffusion process is utilized. There has been known another method of fabricating the Nb.sub.3 Sn type superconductive electrical conductor in which a Nb filament is plated with the third element prior to forming a composite wire to be subjected to a diffusion heat-treatment.
When the superconductive electrical conductor is fabricated in a manner as described above, it is required, in any case, to manufacture an alloy containing the third element. In the third elements, however, there is a metal such as Ti which is extremely active at high temperatures. Thus, in order to alloy Nb, Cu, Sn or the like with such third element, it is necessary to utilize a special dissolution process which is high in cost, such as, for example, a vacuum arc dissolution process or an electron beam dissolution process. This results in the increase in manufacturing cost. In addition, since Sn is extremely low in melting point, it is significantly difficult to alloy Sn with Ti or Ta which is high in melting point. Moreover, there is a fear that the third element causes segregation to occur upon casting, which lowers the quality of the conductor and, therefore, a problem also arises that the addition of the third element causes the workability of the bronze matrix or the Nb filament to be deteriorated.
With the aforesaid method in which the third element is plated on the Nb filament, it is extremely difficult to form the plating layer of the third element of a uniform and a desired thickness and having no surface defect. It is also difficult to work or draw the Nb filament plated with the third element since the plating layer of the third element is liable to crack and to be peeled off the Nb filament.